Modern wind turbines usually comprise a rotor with considerable diameter and width. Mounting a wind turbine could include the steps of transporting the different elements to the site of the wind turbine, assembling the tower sections and the tower, lifting the wind turbine nacelle with a crane and mounting the nacelle on the top of the tower, assembling the wind turbine rotor on the ground, lifting the wind turbine rotor with a crane and mounting the rotor to a low speed shaft extending from the nacelle.
The usual way comprises a number of difficulties which have become more and more severe with the increasing size and width of the wind turbine rotor. In particular, assembly of the wind turbine rotor on the ground is difficult as it requires a large area to be free of obstacles which is substantially horizontal and stable in order to be accessible for the assembly workers and the crane. Furthermore, lifting the rotor to the nacelle is rather complicated as the rotor must be turned by 90° in midair. In other lifting systems it is known to pre-mount the wind turbine hub on the nacelle and then lift each wind turbine blade successively to position it next to the hub and perform the mounting of the blades. Such a system is, for example, described in US 2006/0120809 A1 where the blades are mounted in horizontal orientation, and in US 2006/0147308 A1 where the blades are mounted in vertical orientation. A gripping device for lifting wind turbine blades in a vertical orientation which comprises two clamping jaws is disclosed in US 2005/0019166 A1.
In such systems it is often difficult to release a lifting device from the blade once the blade was mounted to the rotor hub. Some systems require that personnel ascend the lifting device and manually disengage the device from the blade. Other systems rely on the blade being kept in place in lifting the lifting device purely by gravity. After mounting the blade on the rotor hub the device is lowered so that it can be released from the blade width and is then moved horizontally towards the blade tip until it is free of the blade. This requires very skilled crane operation, however, and when carrying out the installation during high wind damage to the blade may occur.